1. Field of the Invention
The present invention relates to a method for preparing difluoromethane (CF2H2) which is used as a refrigerant and the like and is a substitute for a fluorinated gas.
2. Related Art
Methods for preparing CF2H2 (hereinafter referred to as HFC-32 include a process of reducing CF2Cl2 (hereinafter referred to as CFC-12), as described in UK Patent No. 732269; and a process of fluorinating CH2Cl2 (dichloromethane), as described in Japanese Patent Kokoku Publication No. 3004/1967 (corresponding to U.S. Pat. No. 226447), and Japanese Patent Kokai Publication Nos. 225131/1984, 231029/1984, and 231030/1984.
The former process conduct the reduction with hydrogen at a temperature of 400 to 1,000xc2x0 C. in the presence of a Pt, Pt alloy, Cu, Ag or Co catalyst. In the latter process, the catalyst used includes dichromium trioxide, chromium fluoride, aluminum fluoride and a mixture thereof.
HFC-32 draws attention as a substitute for CFC-12 (CF2Cl2, dichlorodifluoromethane) and HCFC-22 (CF2ClH, monochlorodifluoromethane). Accordingly, if HFC-32 can be prepared from the CFC-12 or HCFC-22 raw material, an existing apparatus can be effectively used. In this case, a reaction for reducing the raw material is necessary.
However, a hitherto known reduction method has a high reaction temperature of at least 400xc2x0 C. and results in excess reduction in the case that each of HCFC-22 and CFC-12 is used as the raw material, whereby a large amount of evolved methane is produced so that the selectivity of HFC-32 is low. For example, if the reaction is conducted at the temperature of 720xc2x0 C., the conversion of CFC-12 is 66% and a selectivity of HCFC-32 is only 13.2% (cf. UK Patent No. 732269).
An object of the present invention is to provide a method for preparing HFC-32 from a CFC-12 or HCFC-22 raw material so as to provide both a high conversion and a high HFC-32 selectivity.
The present invention provides a method for preparing difluoromethane (CF2H2, HFC-32) which includes reacting dichlorodifluoromethane (CF2Cl2, CFC-12) and/or monochlorodifluoromethane (CF2ClH, HCFC-22) with hydrogen in the presence of a palladium-based catalyst.
The palladium-based catalyst is preferably a palladium (Pd) catalyst; or a catalyst in which at least one metal selected from the group consisting of vanadium (V), zirconium (Zr), calcium (Ca), magnesium (Mg), niobium (Nb) and tantalum (Ta) is added to palladium.
The above reaction (hydrogenation reaction) in the present invention is preferably conducted at a temperature of 120 to 400xc2x0 C.
The palladium-based catalyst used in the present invention comprises a carrier and an active metal component. The active metal component is preferably a palladium metal or a combination of a palladium metal with at least one additional metal selected from the group consisting of vanadium, zirconium, calcium, magnesium, niobium and tantalum. An amount of palladium supported in the catalyst is preferably from 0.5 to 5% by weight. A molar ratio of the additional metal to Pd is usually from 0.01 to 4, preferably from 0.1 to 2. Since the large molar ratio does not give a significant effect on a selectivity and gives a decrease of reaction conversion, the molar ratio is preferably at most 4. A size of the catalyst is not limited and is usually from 1 to 6 mm. A powdery catalyst may be used.
The additional metal may be in the form of a salt. A nitrate salt, a metal oxide salt, an oxide and a chloride salt can be used. The carrier may be one usually used in conventional catalysts, such as active carbon and alumina. Since HF may evolve in the method of the present invention, it is undesirable to use a catalyst which has no resistance to HF.
One example of procedure for supporting the additional metal on the carrier is explained hereinafter. However, the present invention is not limited to this example. A salt of an additional metal is dissolved in water. Formalin and a powdery catalyst having Pd supported on active carbon are added to water and aged. The additional metal is dissolved in such amount that the desired molar ratio of the additional metal to palladium is achieved. Then, after water is evaporated, the catalyst is dried in air. Before the method of the present invention, the catalyst may be pretreated at 300-500xc2x0 C. for 0.1-10 hours in a hydrogen stream.
In the reaction of the present invention, a molar ratio of hydrogen to CFC-12 or HCFC-22 is usually from 1 to 10. When the molar ratio is from 1 to 10, the selectivity is not adversely affected and the reaction seldom gives an excessively hydrogenated paraffin compound. The W/F (W: weight of catalyst (g), F: total flow rate of raw material and hydrogen (ml/sec at STP)) corresponding to a contact time is preferably from 0.01 to 10. When the W/F is up to about 10, the W/F gives an effect only on the reaction conversion with a slight change of the selectivity.
The method of the present invention is usually conducted in a gas phase. A reaction temperature is usually from 120 to 400xc2x0 C., preferably from 200 to 300xc2x0 C. A reaction pressure is usually from 1 to 10 atm, preferably from 1 to 5 atm. According to the present invention, when the reaction temperature is from 200 to 300xc2x0 C., the reaction gives the result that a conversion from CFC-12 is 91% and a selectivity to HFC-32 is 81%.
In the reaction of the present invention, a raw material is either CFC-12 or HCFC-22; or combination thereof.
According to the method of the present invention, the reaction of CFC-12 and/or HCFC-22 with hydrogen in the presence of the palladium-based catalyst at the temperature of at most 400xc2x0 C. gives a higher conversion and a higher HFC-32 selectivity.